Bonding stiff parallel surface panels to a resin core in order to produce a lightweight yet strong structural sandwich has been known for a long time. Furthermore, curable resins that provide a range of densities and strengths, particularly those resins that provide a high strength-to-weight or stiffness-to-weight ratio, have been used to produce structural sandwiches. Long, U.S. Pat. No. 4,013,810, discloses one such sandwich in which the resin core contains a mixture of resin and hollow glass microspheres. The resins disclosed by Long can be molded into complex shapes suitable for use in aircraft flooring and engine nacelles. The sandwich panels disclosed by Long are composed of a compression molded thermoplastic resin fabricated under conditions of high temperature and pressure and provide some capability for postforming. Structures formed as described by Long are relatively strong and lightweight but have a low compression strength of approximately 1,000 psi.
Lightweight resins such as epoxy or polyester resins having hollow organic or inorganic microspheres incorporated therein have been suggested for use in several structural applications. Massey, U.S. Pat. No. 3,917,547, discloses polyurethane foams that have hollow silica microspheres incorporated in an organic polymeric matrix having improved compression strength. Similarly, both carbon and phenolic microspheres have been incorporated in a variety of resins to produce lightweight materials having good insulation properties. Resins that incorporate metal coated organic and inorganic particles or spheres to enhance electroconductivity or to prevent electromagnetic interference (EMI) and provide radio frequency interference (RFI) shielding are also known. For example, U.S. Pat. No. 4,496,475 describes an electroconductive body suitable for use as termination elements for capacitors fabricated from resins containing at least 10% by weight silver distributed between silver particles and inorganic silver coated spheres. U.S. Pat. No. 4,566,990 discloses conductive thermoplastics in which fibers of glass or graphite are coated with aluminum, copper, silver, nickel, iron or alloys thereof. These metals are present in from 8 to 12% by weight and effectively shield electronic equipment sensitive to EMI/RFI.
Many aerospace structures, such as selected elements of aerodynamic surfaces including wing flaps and ailerons, are now composed of composite structures. Current methods for fabrication of composite structural components require that opposing surfaces of these elements be formed together, normally in an abutting relationship. Because these elements are composed of composites, the tolerances to which these parts can be manufactured are not held to as strict a tolerance as is available for machined metal parts. Furthermore, one or more of the surfaces of composite components are often irregular. As a consequence, when an aerospace structure is assembled from two of these composite structural components with their smooth aerodynamic surfaces facing outward, an irregular gap will exist between the two mated surfaces. This gap in the final aerospace structure is unacceptable because localized overstressing is caused when, for example, the parts are riveted together. Filling the gap between the surfaces has been achieved; however, current methods have several drawbacks. For example, one method involves hand-forming a solid shim from a material such as aluminum. Another method involves removing plys from a shim made of a laminated polymer until the required contour and shape are achieved. Both of these methods require trial fitting, removal, hand-shaping and refitting of the shim until a satisfactory contour is achieved. These methods are labor intensive, time-consuming and produce a nonstandard product that varies in quality with the skill of the mechanic. Often the contour of the gap is complex, that is, it varies in thickness in both directions simultaneously and the gap cannot be adequately filled by either method. Any gap remaining after shimming may result in localized distortion and stressing of the composite structure when the fasteners are installed.